1. Field of the Invention
This invention relates to fluid check valves, specifically to an improved prosthetic mechanical heart valve intended for replacement of human heart valves.
2. General
Since the 1960's, surgical replacement of diseased natural heart valves has become safer and more practical. These prostheses, which function as fluid check valves, are actuated by the pumping action of the heart. It is essential that in order to function as long-term life-support devices, such valves must satisfy the following requirements: complete biocompatibility, high fluid efficiency, extreme reliability, outstanding durability and resistance to wear.
The valve must be constructed to withstand approximately 40 million opening and closing cycles per year under the unceasing pumping of the human heart. Blood flow through the valve must not be restricted and the valve function must under all circumstances not be destructive to the blood that is pumped therethrough. The valve prosthesis must not place unnecessary stress on the heart. Furthermore, the valve must be designed in such a way as to minimize the often fatal side effect of thromboembolism, or the formation and liberation of a blood clot from the prosthetic valve. It is currently a recognized therapy that all patients receiving mechanical heart valve replacements must take anticoagulant drugs to reduce the potential of a thromboembolic event.
3. Description of Prior Art
Heretofore, a wide variety of mechanical heart valves have been designed and developed around this area of bioengineering endeavor. Reference is made hereby to an important body of this literature. Nevertheless, existing prosthetic valves fall short of an ideal substitute for the healthy, natural heart valves, and work continues to improve the functioning of these valves to make them even more reliable, efficient and resistant to the formation of blood clots. Most notably, none of the existing devices work like the natural valves, instead placing the valving mechanisms inside the valve body itself.
All of these devices function as one-way check valves, and have one or more occluder members to seal the valve, preventing reverse flow. Devices now in use generally place the occluding members and the supports or restraints for those occluders in the path of the blood flow when the valve is open. This design aspect increases the resistance to fluid flow by reducing the effective unobstructed orifice area compared to the tissue implant diameter, which seriously reduces the hydraulic efficiency of the device. It further prohibits the possibility of tissue ingrowth, which, while it could greatly reduce the thromboembolic potential of the device, would interfere with free movement of the occluder(s).
The first successful mechanical valves were "ball-in-cage" valves, and were made with a mobile poppet that was pushed to and fro with the blood pumped through the heart. U.S. Pat. No. 3,365,728 to Edwards et al (1968) discloses a valve where the sealing action is accomplished by a soft poppet which is restrained by a metallic cage. The literature has shown that in actual use, blood clots form on the stationary cage and that the resistance to forward flow around the poppet is high. While the poppet is displaced downstream when blood is flowing through the valve, this design is still obstructive to free flow as the blood must still change directions and flow around the ball.
The next major advance in this technology was the introduction of the "tilting-disc" valve, a device where a discoid occluder was suspended in a ring and rocked open and closed with blood flow. U.S. Pat. No. 3,824,629 to Shiley (1974) discloses valve with a tilting saucer-shaped occluder suspended in a cage. This design, while an improvement over the ball valves, still placed the occluder in the flow path during forward flow, resulting in a significant pressure drop across the valve. Thrombosis problems (total valve obstruction by blood clots) have been reported with this type of valve.
In an effort to reduce the pressure drop across the valve, the occluder was split into two pieces, each being separately pivoted. U.S. Pat. No. 4,689,046 to Bokros (1987) discloses a representative example of these twin-occluder or "bileaflet" valves with the occluder pivots riding in depressions within an annular body. This design represented a further improvement in flow resistance reductions, but the occluders are still in the flow stream when open. This type of design still exhibits the problems of blood clots forming in the stagnant areas of the pivot depressions. Further, these designs concentrate the bearing and rotational wear in the pivot into very small areas, and extremely hard materials, such as pyrolytic carbon are required to effect adequate durability of these prostheses. Besides being very hard, pyrolytic carbon is also brittle and fractures of this material which caused failure of this type of valve have been reported.
There are also other problems common to existing designs in current use. The valves with a mobile central leaflet or leaflets have been shown to become stuck on surgical sutures or subvalvular tissues within the heart. This extrinsic interference is a result of a foreign body becoming jammed between the disc or leaflets and the orifice ring, and results in failure of the valve to function. Therefore, tissue ingrowth into the valve must be prevented as it would interfere with the operation of the valve. This is a disadvantage, because if a prosthesis could be covered with the heart's own lining, it would be as thrombus-free as the heart itself.
Also, valves with centrally-mounted occluders, particularly the "tilting-disc" valves, must be precisely oriented within the heart to produce the best flow characteristics. Finally, all of the mechanical heart valve prostheses in use today still require the recipient to take anticoagulant drugs to minimize the risk of thromboembolism. This is due to a design feature in each of these valves that creates a region of stagnation where blood clots can nucleate. In some designs, this is in a recess in the pivot region, in others it is behind a non-mobile cage or supporting structure of the valve.
Few exceptions to locating the occluder means out of the field of flow are found. U.S. Pat. No. 3,589,392 to Meyer (1971) discloses a split leaflet valve where the leaflets are hinged at the outer edge, but the leaflet assembly is contained in a thick external tube that would greatly reduce the ratio of flow area to tissue annulus implant diameter. The leaflets are very high in profile and therefore likely to contact structures within the heart on opening. Also, the leaflets are allowed to open to 90.degree. and reverse flow through the valve would run parallel to the fully open leaflets, not necessarily creating enough force on them to make them swing closed. Finally, if the valve was oriented the slightest bit out of exact alignment with the flow stream, one of the leaflets would positively be held open by reverse flow defeating its function as a check valve.
U.S. Pat. No. 4,114,202 to Roy et al (1978) shows an embodiment where the leaflets are hinged at the outboard edge, but the leaflets remain in the throat area of the thick annular body of the valve, still obstructing flow. U.S. Pat. No. 4,178,638 to Meyer (1979) discloses a fluid check valve that is hinged at the outer edges, but the hook forming the hinge requires the use of a recessed well, creating a large, vacant area of stagnation which would invite the formation of blood clots. The hinge design here concentrates wear on the tip of the restraining edge of the hook, and is therefore unsuitable for the countless opening and closing cycles exerted on a heart valve. Also, the stationary downstream ring required to restrain the leaflets in the cardiac application would further invite the formation of blood clots as seen with the ball valves, and the use of soldered assemblies has not provide suitable for demanding implant applications.
U.S. Pat. No. 4,263,680 to Reul et al (1981) discloses a single leaflet valve with an external hinge, but the distance the full diameter occluder must traverse between fully open and fully closed would cause backflow through the device to be unacceptably high. Also, there is nothing to restrain the occluder member from overrotating beyond 90.degree., and therefore not closing, defeating its function as a check valve. Further, protrusion of the full diameter edge-hinged disc into the chambers of the heart would likely contact structures inside the heart, damaging the heart tissues and obstructing valve function.